1. Field of the Invention
The present invention relates to an apparatus of improved performance for demodulating frequency-modulated optical communication signals and magnetoelectric signals.
2. Description of the Prior Art
Of the digital data recording techniques known heretofore, NRZ (non-return-to-zero) system has been practical for a long time. In this system, however, there exist some disadvantages including a considerable variation that may occur in recording frequency depending on the pattern of data to be recorded, the necessity of a timing signal for demodulation of the data in addition to the primary data signal to be recorded, and strict requirements relative to temporal phase deviation (skew) between the data signal and the timing signal. Under such circumstances, an FM system adapted to achieve improved digital data recording is now widely applied for the purpose of eliminating the above mentioned disadvantages.
The FM system, which is a kind of self-clocking modulation, is advantageous in comparison with the foregoing NRZ system since there never arises a problem with regard to skew and so forth since the data is transmitted in the form of a composite signal obtained by mixing the data and a timing signal (which is a clock pulse) with another signal. In demodulating the FM signal, however, when any large variation exists in a bit period of the signal, it is impossible to attain accurate demodulation. In an attempt to eliminate such defects, some improvements have been previously proposed as disclosed in the U.S. Pat. Nos. 3,902,129, 3,949,313, 3,962,726 and 4,540,947.
Particularly, in a system proposed by the applicant and disclosed in said U.S. Pat. No. 4,540,947 pulse train of 64(d)(2/3) with every third one of successive clock pulses from an oscillator as shown as NEG(2/3), 64(d)(2/3) in FIG. 1. By this means the FM signal is demodulated. However, a peak shift caused by high density of magnetic record ensure an accurate demodulation of FM signals is difficult to attain. For this reason it is required to prepare an appropriate counter-measure. As shown in FIG. 2 a signal which will cause a magnetic inversion on magnetic recording media and a read pulse thereof are shown. In the figure reference letter and numeral S-1 designates a written pulse, reference letter and numeral S-2 designates a magnetic inversion signal and reference letter and numeral S-3 designates a read pulse thereof. This read pulse is generally called an independent pulse when regenerated. A regenerated pulse signal is recorded by means of successive magnetization inversion on magnetic recording media and simulatively evaluated by means of synthesization of said independent pulse. In other words, when successive magnetization inversions are conducted a continuous regenerated pulse is obtained by means of synthesizing an independent pulse of dashed line as shown in FIG. 3 and said regenerated synthesized pulse as shown with said solid line is located with a peak different from the independent pulse. This phenomenon is called a peak shift. In the figure reference mark and letter .DELTA.T designates a peak shift and reference letter and numeral S-4 designates a regenerated pulse. In FIG. 4 a typical example of said peak shift is shown. In the figure reference letter and numeral S-5 designates a signal of reformed pulse and reference letter and numeral S-6 designates a first flip flop output (mask signal). In FIG. 4 it is required to demodulate the FM signal as accurately as much as possible in spite of existence of a peak shift (.DELTA.T).